The celebrated and controversial Spanish human rights investigator Baltasar Garzón is to lead the defence of WikiLeaks founder Julian Assange as he fights extradition from Britain to Sweden where he is wanted for questioning over rape allegations.

Garzón, who was disbarred as a judge in Spain in February, said he would lend Assange the weight of his knowledge of international human rights and extradition law in a case that he denounced as "arbitrary and baseless".

He travelled to London to meet Assange, who has been taking refuge in the Ecuadorean embassy since 19 June.

Garzón, who was the investigating magistrate when the former Chilean dictator Augusto Pinochet was arrested in London on an extradition warrant to face genocide charges in Spain, said he believed Assange was the victim of obscure international political manoeuvring. "There is clear political intentionality behind this affair, which explains his current situation," added Garzón on Wednesday.

"Obviously this is not just about his future but his physical and psychological stability and It is only right that Assange should be protected by the same rights as those of any other citizen."

"Assange has not rebelled against any jurisdiction, given that he respects the action of the law, but he – and we – are seriously worried about what will happen to him because his situation is becoming political as a result of the great work done by his organisation when it comes to denouncing corruption," he added.

"That cannot be the reason for a judicial process that appears, and which I believe we can show, is arbitrary and totally baseless," he stated, according to the Cadena SER radio station.

"It does not seem right that a single person should be under such pressure from governments," referring to both Sweden and Britain. "I believe that Assange … is in a situation that is an attack on his human rights."

A statement posted on the Support Julian Assange website http://www.support-julian-assange.com/tag/baltasar-garzon in the names of both Garzón and Assange confirmed the news. "The Spanish judge, lawyer, and international jurist, Baltasar Garzón, will lead the legal team representing Julian Assange and WikiLeaks," it said.

The two men had discussed a new legal strategy to defend Assange and WikiLeaks from "the existing abuse of process".

Its aim included showing "how the secret US processes against Julian Assange and WikiLeaks have compromised and contaminated other legal processes, including the extradition process ."

"The judge has expressed serious concerns regarding the lack of safeguards and transparency with which actions are being taken against Julian Assange, and the harassment he is being subjected to which has irreparable effects on his physical and mental wellbeing," the statement added. "The threats against his person are further aggravated by the complicit behaviour of the Swedish and UK governments, who are wrongfully abrogating his rights."

Garzón's career as an investigating magistrate in Madrid in effect came to an end five months ago when the supreme court disbarred him for wiretapping conversations between defence lawyers and their clients in a corruption investigation involving the Spanish prime minister Mariano Rajoy's People's party.

He was given an 11-year suspension as supporters claimed he was the victim of a conspiracy to bring down one of the world's best-known human rights investigators, who had successfully pursued henchmen working for Argentina's military juntas.